When a box appears asking for a user name and password it almost always means that there is a problem at the Plusnet end and the computer that is supposed to validate these credentials is not responding as it should. The correct response is to do nothing and wait an hour or two until Plusnet fix their fault. But 9 people out of 10 do not respond this way. They enter their user name and password and when this 'doesn't work' they try something different and eventually give up. Since the computer remembers the last entry in the box, this is likely to be wrong so the situation never recovers.
Go to the Plusnet web site and login to your account. Whatever user name and password combination gets you in is most likely to be the same combination your email program requires. Having validated that you have the correct user name and password, enter these into the pop-up box.
